Re: [PATCH] libstdc++: Fix infinite loop in std::istream::ignore(n, delim) [PR93672]

[Iain Sandoe <idsandoe@googlemail.com>]

> On 4 Apr 2024, at 16:29, Jonathan Wakely <jwakely@redhat.com> wrote:
> 
> I would appreciate more eyes on this to confirm my conclusions about
> negative int_type values, and the proposed fix, make sense.
> 
> Tested x86_64-linux.
> 
> -- >8 --
> 
> A negative value for the delim value passed to std::istream::ignore can
> never match any character in the stream, because the comparison is done
> using traits_type::eq_int_type(sb->sgetc(), delim) and sgetc() never
> returns negative values (except at EOF). The optimized version of
> ignore for the std::istream specialization uses traits_type::find to
> locate the delim character in the streambuf, which _can_ match a
> negative delim on platforms where char is signed, but then we do another
> comparison using eq_int_type which fails. The code then keeps looping
> forever, with traits_type::find saying the character is present and
> eq_int_type saying it's not.
> 
> A possible fix would be to check with eq_int_type after a successful
> find, to see whether we really have a match. However, that would be
> suboptimal since we know that a negative delimiter will never match
> using eq_int_type. So a better fix is to adjust the check at the top of
> the function that handles delim==eof(), so that we treat all negative
> delim values as equivalent to EOF. That way we don't bother using find
> to search for something that will never match with eq_int_type.

Is the corollary to this that a platform with signed chars can never use a
negative value as a delimiter - since that we always be treated as EOF?

- I am not sure it there’s an actual use-case where that matters, but,
Iain

> 
> The version of ignore in the primary template doesn't need a change,
> because it doesn't use traits_type::find, instead characters are
> extracted one-by-one and always matched using eq_int_type. That avoids
> the inconsistency between find and eq_int_type.
> 
> libstdc++-v3/ChangeLog:
> 
> 	PR libstdc++/93672
> 	* src/c++98/istream.cc (istream::ignore(streamsize, int_type)):
> 	Treat all negative delimiter values as eof().
> 	* testsuite/27_io/basic_istream/ignore/char/93672.cc: New test.
> ---
> libstdc++-v3/src/c++98/istream.cc                 |  5 ++++-
> .../27_io/basic_istream/ignore/char/93672.cc      | 15 +++++++++++++++
> 2 files changed, 19 insertions(+), 1 deletion(-)
> create mode 100644 libstdc++-v3/testsuite/27_io/basic_istream/ignore/char/93672.cc
> 
> diff --git a/libstdc++-v3/src/c++98/istream.cc b/libstdc++-v3/src/c++98/istream.cc
> index 07ac739c26a..aa1069dea07 100644
> --- a/libstdc++-v3/src/c++98/istream.cc
> +++ b/libstdc++-v3/src/c++98/istream.cc
> @@ -112,7 +112,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>     basic_istream<char>::
>     ignore(streamsize __n, int_type __delim)
>     {
> -      if (traits_type::eq_int_type(__delim, traits_type::eof()))
> +      // sgetc() returns either (int_type)(unsigned char)c or -1 for EOF.
> +      // If __delim is negative, then eq_int_type(sgetc(), __delim) can only
> +      // be true for EOF, so just treat all negative values as eof().
> +      if (__delim < 0)
> 	return ignore(__n);
> 
>       _M_gcount = 0;
> diff --git a/libstdc++-v3/testsuite/27_io/basic_istream/ignore/char/93672.cc b/libstdc++-v3/testsuite/27_io/basic_istream/ignore/char/93672.cc
> new file mode 100644
> index 00000000000..6d11f5622c8
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/27_io/basic_istream/ignore/char/93672.cc
> @@ -0,0 +1,15 @@
> +// { dg-do run }
> +
> +#include <sstream>
> +#include <testsuite_hooks.h>
> +
> +int main()
> +{
> +  std::istringstream in("x\xfdxxx\xfex");
> +  in.ignore(10, std::char_traits<char>::to_int_type('\xfd'));
> +  VERIFY( in.gcount() == 2 );
> +  VERIFY( ! in.eof() );
> +  in.ignore(10, '\xfe');
> +  VERIFY( in.gcount() == 5 );
> +  VERIFY( in.eof() );
> +}
> -- 
> 2.44.0
>